01 November 2011
Ground source heat pumps (GSHP) gain importance world-wide with respect to energy efficiency in heating and cooling operation. The ground acting as a seasonal store offers the possibility of damping the effects of the outside air temperature fluctuations, in colder climates it enables monovalent heating operation of the heat pump, and for utilities it is – compared with outside air operated heat pumps – a tool for demand side management measures.
In the last decades, heat pumps have acquired fundamental shares in markets such as Japan and the United States. Other markets including Europe, with the exception of Sweden and Switzerland, and other continental zones are struggling to develop the implementation of heat pump technologies as basic heating and comfort cooling devices.
Since the Eighties heat pump units and the components used like advanced compressors and flat plate heat exchangers, respectively, have been improved significantly. The development of heat source systems and heat sink systems on the one side and the system approach on the other side took much more time, and the development in the direction of highly efficient systems is still going on.
Within the framework of the IEA Heat Pump Programme 3 Annexes on ground coupled heat pumps have been carried out,
• Annex 2: Vertical Earth Heat Pump Systems,
• Annex 8: Advanced in-ground Heat Exchange Technology for Heat Pump Systems, and
• Annex 15: Heat Pump Systems with Direct Expansion Ground Coils,
showing the importance of this technology for both cooling and heating operation. A completely different use of the ground happens in the case of large systems with both cooling and heating demand. In such a case natural recharging of the ground no longer works, i.e. the heat extracted through heating operation has to be recharged artificially and an excellent solution is to use heat removal from cooling operation. However, taking this type of utilisation of the ground, the ground becomes a store, and the temperature changes in this store are the result of heat extraction/heat removal over the year.
In this context the Energy Conservation through Energy Storage Implementing Agreement has carried out
• Annex 13: Design, Construction and Maintenance of UTES Wells and Boreholes.
Taking these 4 Annexes both Programmes came to the result that:
• Ground coupled systems offer in the case of heat pumping systems stable operating conditions.
• Ground-coupled heat pumps are presently dominating the heating-only heat pump market in Europe.
• Ground coupled heat pumps have been also identified as an interesting and energy efficient solution for the heating and cooling market in North America; there is also a large interest from other countries like Japan and China in this technology.
• In the case of large systems the ground can be used as a store, which offers in the case of heating and cooling operation at least improved conditions at the beginning of both the heating and the cooling season. Additionally, direct cooling becomes possible.
The work of this IEA Heat Pump Programme Annex 29 includes a state of the art review and a market study on GSHPs, a matrix of GSHP applications depending on climate, ground conditions and applications, a study on how to improve technical performance and cost of GSHP systems, the identification of market barriers and innovative approaches to increase the acceptance and with the acceptance the market share of this highly efficient technology.
Heat pumps are an old technology, which has not been extensively used as long as both energy prices and the efficiency of electricity generation have been low. The oil crises have changed this situation, and now Kyoto is a further reason for the increasing market deployment of this technology. Based on recent developments, the following conclusions can be drawn:
• Heat pumps offer the possibility of reducing energy consumption significantly, mainly in the building sector, but also in industry. Basic second law thermodynamics show the advantages: while a condensing boiler can reach a primary energy ratio (PER) of 105 % (the theoretical maximum would be 110 % based on the lower calorific value), heat pumps achieve 200 % and more, with hydro or wind energy even 400 % and more.
• The drive energy is most commonly electricity, and for the future improved power generation systems based on renewables and fossil fuels have to be taken into consideration. The efficiency of gas-fired combined-cycle power plants available on the market is presently about 58 %, with oil as fuel similar values are possible. Ground-source (“geothermal”) heat pumps combined with low-temperature heat distribution systems achieve seasonal performance factors (SPFs) of 4 and higher, which means PERs of 220 to 280 %.
• Heat pumping technologies, i.e. refrigeration, air conditioning and heat pumps, have undergone and are undergoing several changes in working fluids and design. However, the efficiency today is generally better than before these changes and keeps rising. Thus, not only the environmental effects of the working fluids are being reduced, but also the effects of power plants producing the drive energy for the heat pumps – due to higher SPFs, higher power plant efficiencies, and an energy sources mix with lower CO2 emissions. Therefore, the TEWI (Total Equivalent Warming Impact) is reduced significantly. Phasing out HFCs is not so much a problem of technology, it is a problem of efficiency. Applications will remain where “safety” refrigerants, i.e. non-toxic and non-flammable fluids, have to be used also in the future. But the natural refrigerants such as ammonia, propane, CO2 and some others will get an increasing share. The question of environmental acceptability seems to be solved. The main point, however, is efficiency, and the energy requirement has to be minimized.
• Ground-coupled heat pumps gain importance world-wide with respect to energy efficiency in heating and cooling operation. The ground acting as a storage offers the possibility of damping the effects of the outside air temperature fluctuations, in colder climates it enables monovalent heating operation of the heat pump, and for utilities it is a tool for demand side management measures. New developments like improved heat pump units, advanced direct-expansion heat pumps or heat pumps combined with heat pipe based vertical probes show that there is still room for new ideas, which may be necessary for being competitive and successful in the future.
• Direct-expansion ground-source heat pumps already achieve SPFs between 4 and 5, if the building standards are being kept and the design of the overall system has been made carefully. Such high SPFs are the work of highly skilled system constructors; they do not sell heat pump units, they sell systems. The choice of the refrigerants they use, i.e. presently R-410A, propane and CO2 for heat pipes, is motivated by efficiency, reliability, environmental considerations, safety and regulations. Direct-expansion systems, either with horizontally installed collectors or with bore holes down to 60 m, show increasing sales figures, and they dominate the Austrian market. But one has also to consider the building stock, and retrofitting this building stock will become the heat pump market of the future. New developments like improved heat pump units, advanced direct-expansion heat pumps or heat pumps combined with heat pipe based vertical probes show that there is still room for new ideas, which may be necessary for being competitive and successful in the future.
• The choice of an air conditioning system for a commercial building depends on the climatic conditions, on the building and on the utilisation of the building. In the meantime the design of the building became the main factor concerning energy consumption. The air conditioning system has the task to compensate external and internal loads and to provide hygienic conditions and year round comfort for the customers. Additionally the air conditioning system offers possibilities to carry out this task with a minimum amount on energy by shifting heat from spaces, which have to be cooled to spaces, which have to be heated at the same time. Using the ground as a store additionally heat and cold can be stored to a certain extent and used for providing cold without additional energy input, i.e. for direct cooling, and for increasing the heat source temperature for heating. Using low-ex systems these effects can be further increased.
• Sorption systems – absorption, adsorption and DEC systems – also gain importance. The efficiency of sorption units has been improved significantly by introducing welded flat plate heat exchangers for reducing heat transfer losses.
• With highly efficient systems the advantages of thermodynamic heating and cooling can be demonstrated and used for reducing the energy demand significantly.
The potential for reducing CO2 emissions assuming a 30 % share of heat pumps in the building sector using technology presently available is about 6 % of the total worldwide CO2 emission. With advanced future technologies in power generation, in heat pumps and in integrated control strategies up to 16 % seem to be possible. Therefore, heat pumps are one of the key technologies for energy conservation and reducing CO2 emissions.